INTRODUCTION

As is known, the performance indices of blast furnaces are preconditioned in many respects by the method used to charge burden into them. These methods can be divided into three main classes. Method 1 is the one when burden is charged in annular batches through a two-bell apparatus. By method 2 stock is charged as a single flow through the chute-type chargers. And the last is method 3, which is a multi-flow stacking of burden material through rotary charging units. Usually, the chute-type apparatuses used to be considered as the most advanced from the technological point of view. However, in this paper a different viewpoint is expounded, which is corroborated by the industrial test results obtained from application of the bell-less rotary charging unit. The inferences in favor of the rotary design of such apparatuses as compared with the chute-type ones are drawn.

Fig.1 BRCU

DISCUSSION

In August and October 2006 two blast furnaces (1681 m3 each) were effectively commissioned at JSW and JSPL steel plants (Jindal Group, India). Both the furnaces were equipped with bell-less rotary charging units (BRCU). A while ago in papers [1;2] the principal differences between the rotary method of burden charging and chute-based charging technique were explained. Yet, it may be noted that the salient features of the rotary method of burden charging are as follows:
- soft stacking of burden material in thin layers, which wards off deformation of profiles of the lower layers and their excessive compacting,
- multi-layer stacking of burden by 5 vanes of the rotor, as a result of which burden material is averaged in terms of its chemistry and size distribution. One batch of material would be stacked in 30 to 50 layers,
- flexible control of material distribution in radial direction, which is achieves by varying the speed of the rotor.

The combination of these characteristics accounts for the advantage of the rotary distribution of burden as compared with that through chutes. This affirmation is substantiated by analyzing the performance of BF-2 at JSPL. The results are cited in this paper.

The general view of RCU is shown in Figure 1. The BRCU makeup is as follows: receiving funnel, upper and lower banks of valves, transfer hopper, burden gate with compensator, rotor, central gearbox, and rotor remote drive. Inside the central gearbox there is the main bearing with gear ring, which is responsible for the rotation of the rotor. Inside the central gearbox enclosure , at its bottom there is a stationary cooling arrangement. The performance of BRCU mechanisms, their position and preset distribution of stock are controlled through ACS “ROTOR”,which is integrated into the blast furnace ACS.

To assist the operators in analyzing and optimizing the charging modes a simulating model was developed (hereinafter SM) for shaping and visualizing the descending layers of burden. It is the results of the physical model experiments that were used as the main component of the model. The experiments with the help of the physical model were used to determine the parameters of stock distribution along the top radius, depending upon the charging mode, provided by the rotor. The factors that were taken into account

Fig.2 The interface of the simulating model

as exerting influence upon the distribution of material in experiments were speed of the rotor, height of the stock line, quantity and quality of material, rates of material, delivered to the rotor. In simulating the stacking of layers the account was taken of impacts made by the stock descending rates curve along the furnace radius, angles of repose for various materials, profiles of the preceding layers, whereupon the desired layer was stacked, volume and type of the charged material.

The interface of the simulating model is shown in Figure 2. The first diagram of SM depicts the visualization of stock layers positions after having charged one cycle consisting of 8 batches. Below, in diagram 5 there is a curve of material descending rates. In this particular case the descending rate happened to be uniform along the radius and in fact it was a relative unit along the whole radius. Diagram 2 shows variations in ore-to-coke ratio (as summarized for the whole cycle) along the furnace radius. Below, in diagram 4 there are correlations between ore bearing material and coke in the over-all volume of stock in one cycle. Diagram 3 shows how coke is distributed along the furnace radius when it is loaded into the centre on a flat surface. In this case the diagram depicts the distribution of the first batch in the cycle, which contains coke, as it is loaded into the furnace centre. The diagram reflects the results obtained from experiments in the physical model. Diagram 7 illustrates the preset modes of discharging from the rotor, for each batch in the cycle. The conditions of stacking material on the burden surface inside the furnace are represented in Table6 of the Interface. Thus, after presetting the desired mode of charging and determining the conditions of stacking material on the burden surface one can plot the layer-by-layer diagrams of material layers in the furnace top, detecting at that the variations in ore-to-coke ratio along the furnace radius.

Figure 3 shows examples of various charging patterns in the furnace. In the lower raw in tables there are set-points for each pattern. Here and henceforth the jobs to load stock on a particular ring mean the setting of the rotor speed in such a way that it would place the stock ridge onto the desired ring. Figures in the jobs table are the percentage of time spent for the outflow of material on a corresponding ring. The whole span of the top is split into six equal-in-area rings.

While using System 1, which consisted of 8 batches, the first batch of coke was loaded upon the second ring. The second batch, which consisted of ore bearing burden components, was loaded upon ring 4, the third batch of coke was distributed into three rings at ratio 40-10-50 judging by the discharge time, on rings 3-4 and 5 respectively. The pattern of distribution of the remaining 5 batches one can see also in the job matrix. The visualization of layers stacking in System 1 is represented in the left upper diagram 1, the lower diagram shows how ore-to-coke ratio varied along the top radius. Here one can see that the ore-to-coke ratio in the furnace center was 1.6 t/t, while in periphery closer to the furnace walls it was about 5.8 t/t. The maximum of this ratio happened to be 6.0 t/t and it was observed in the area of ring 5. Obviously, that such a charging system would have resulted in the furnace run with its center being open and it can be recommended for practical use.

In the left part of Figure 3 there is System 3, which consists of 6 batches of stock. At that coke in batches 1,3,4 and 6 would be always loaded on the central ring, while batches 3 and 5 distributed on rings 2,3 and 4 in ratio 20-40-40. In fact System 3 consists of two cycles, each of them consisting of three batches. The diagram of variations in ore-to coke ratios for this system shows that in this case the furnace center would be opened with ore-to-coke ratio being 0.5 t/t and in periphery it was 3.8 t/t. The maximum ore-to-coke ratio in this case was in the third ring zone and came up to 5.45 t/t. However, as can be seen from the job matrix, the center of the furnace can be loaded additionally by redistributing ore bearing material in batches 2 and 5 towards increased charging on to ring 2 and decreased charging on to rings 3 and 4. Charging system 2 in terms of ore-to-coke ratios distribution holds an intermediary place between systems 1 and 3.

To quantify the distribution of ore-to-coke ratios along the furnace radius we suggested digital criteria, that are calculated by the following formulae:

Ñharging System 1 Charging System 2 Charging System 3

Material *

Ring number

Ring number

Ring number

1

2

3

4

5

6

1

2

3

4

5

6

1

2

3

4

5

Coke

100

100

100

Coke

Ore

100

50

50

20

40

40

Ore

Coke

40

10

50

80

20

100

Coke

Ore

100

100

100

Coke

Coke

50

50

80

20

20

40

40

Ore

Ore

100

100

100

Coke

Coke

100

Ore

100

Fig.3 Examples of charging system

*In the examples for charging system 1 weight of coke portion was C=9100kg , weight of ore portion was O=37000 kg, for system 2 C=8100, O=36000 kg, for system 3 C=5500 kg, O=38000 kg.

Where Cr1, Cr2 and Cr3 are criteria depicting the radial distribution of ore-to-coke ratios along the furnace radius, OR(4-6) - ore-to-coke ratio in zone between rings 4 and 6, OR(1-3) - ore-to-coke ratio in zone from ring 1 to 3, OR(5-6) - ore-to-coke ratio in zone from ring 5 to 6, OR(2-4) –ore-to-coke ratio in zone from ring 2 to 4, OR(1) – ore-to-coke ratio in the furnace center in zone of ring 1. Thus, the suggested criteria would make it possible to quantify the asymmetry in the distribution of ore-to-coke ratios in the radial direction. The suggested criteria can be used for developing various algorithms to control the charging process.

From the examples in Figure 3 one can draw the following observations. In all three cases the character of distribution of ore-to-coke ratios along the radius used to be approximately the same. The ore-to-coke ratio trended to increase from the cener to the periphery. At that there was no sign of correlation between the stock line profile and ore-to-coke distribution curve. So, in system 3 the inclinations of these curves was in different directions. The peak of ore-to-coke ratios in the radial direction in its position did not coincide with that of the stock ridge on the burden profile. For instance, while using systems 1 and 3 the profile ridges would be seen just near the furnace walls, at the same time the ore-to-coke ratio peaks happened to be in the zone of rings 5 and 3 respectively. All these testify to the fact that it is quite a complicated thing to predict the character of ore-to-coke ratios by the stock line profile, without taking note of the capability of material to pour over and become spontaneously redistributed. For instance, in the charging system 1 the last batch of coke was loaded on to ring 6, however, after the visualization of the burden layers setup one cannot see if there is a tangible increase of coke in the periphery in a layer corresponding to this batch. Due to the pouring of coke over towards the center it became redistributed almost completely. The simulating model , suggested by us, can widen to a greater degree the possibility to control the furnace charging process.

In paper [3] there is a more detailed description of possibilities to optimize the ironmaking technology with the help of the suggested criteria.

In the course of running BRCU in the ironmaking process, BF-2 personnel had developed a home-made charging system , which is given in Table 1.

Charging System at JSPL Table 1

Material

Ring number

1

2

3

4

5

6

1.Coke

100

2. Ore

20

80

3.Coke

30

30

30

10

4. Ore

20

80

5.Coke

30

20

30

20

6. Ore

20

80

7.Coke

30

30

20

20

8. Ore

20

80

The visualization of this system is given in Figure 2. Diagram 6 gives criteria for the distribution of ore-to-coke ratios. This system has been in use for quite a long time. In case of deviations in heat parameters from those preset the operators would resort only to the redistribution of coke in ring zones 2-5 along the furnace radius when batches 3,5 and 7 of the cycle were being loaded. The radial distribution of ore-bearing components and charging of the center with coke in the first batch of the cycle were not altered.

THE IDUSTRIAL TESTING OF ROTARY BURDEN CHARGING

The analysis of the BRCU commercialization is made on the basis of operating data from BF-2 at JSPL steel plant. It should be noted that BRCU was put into commercial service simultaneously with putting into service a green field blast furnace, equipped with innovative coal dust injection arrangements, slag granulation systems, weighing and conveying equipment, sintermaking plant etc., other innovative equipments and technologies. Besides, in connection with enhancement of hot metal output, new iron ore and coke sources were enlisted. Quite a few engineers were invited to join from outside. In fact, a new team of operators was cast, who had not worked with each other earlier. Naturally, all the above- said factors made the period needed for mastering technologies and achieving production targets in ironmaking longer.
In table 2 there are variations in the performance indices of blast furnace No. 2 at JSPL (hereinafter referred to as BF-2) that took place in the commercialization period and achieving production targets and after that. For the sake of comparison, in the first column of Table 2 there are performance indices of blast furnace “G” at Tata Steel (hereinafter referred to as BF-G) which is equipped with a chute-type charger and used to demonstrate the record performance results in India. These indices we took as a primary standard for reference. Further, while plotting diagrams, a primary standard (each selected BF-G performance index) was assumed as a unit, and the compared index of BF-2 performance was normalized with reference to this index and taken as a fraction of it..

Figure 4 shows the dynamics of variations in some BF-2 indices, that are quite different form those of BF-G. As can be seen from diagrams in Figure 4, the ironmaking technology at BF-2 was different from that at BF_G (red line) in terms of substantially bigger yield of slag. The excessive yield of slag at BF-2 in all periods of time was more then 20%. Slag yield at BF-G was 263 kg/t of hot metal, while at BF-2 it fluctuated within the range of 321-338 kg/t (see Table 2). Concerning the coal dust rate, that difference was even bigger. While putting BF-2 into commercial service (the fist 4 periods) the coal dust rate amounted to approximately 30% of the primary standard reference,. Then it went on growing and in the last periods of commissioning reached approximately 80% of the reference figure (periods 8-10 in Table 2). Si in hot metal at BNF-2 was constantly lower then at BF-G, approximately by 20% and came to 0.55-0.75 %.

Adjustment coefficients Table3

Sr.No.

Item

Unit

Variation

Efficiency

Product.

Coke rate

1

Shutdowns

%

1

-1,5

0,5

2

Blast temperature

C

100

02.ôåâ

-2,2

3

Top pressure

Pos. atm

0,1

1

-0,2

4

Fe in burden

%

1

1,7

-1

5

Coke ash

%

1

-1,3

1,3

6

Raw lime stone

Kg/thm

10

-0,5

0,5

7

-5 mm size in burden

%

1

-1

0,5

8

+80 mm size in burden

%

10

-2

2

9

M10 coke

%

1

-2,8

2,8

10

Sulfur in hot metal

%

0,010

1,000

-1

11

No blast taps

%

1

-0,3

0,2

12

Oxygen in blast

%

1

2,5

0,2

13

Si in hot metal

%

0,1

-1,2

1,2

14

Mn in hot metal

%

0,1

-0,2

0,2

All the indices represented in Figure 4, would exert a considerable influence upon economics of the blast furnace performance and they must be taken into account in the comparative analysis through correcting coefficients. The heat indices were affected by other factors also though to a lesser degree but rather tangibly, therefore they must be taken into account in analysis. Table 3 gives adjustments to take care of impacts by various factors on coke rates and productivity of the blast furnace, as it is assumed in this paper.

However coke rate in the first four periods of commissioning happened to be higher approximately by 20% then at BF-G, which could be accounted for by a lesser amount of injected coal dust ( about 30% of coal dust rate injected at BF-G). Further, as the coal dust injection was increasing at BF-2, the coke rate was dwindling and by the end of the commercialization period the coke rate
became tangibly lower. According to monthly average indices for February and April 2008 , the coke rate at BF-2 was 426 and 421 kg/thm respectively, against 419 kg/thm at BF-G. This figures are somehow higher then those at BF-G (see table 1), but at that the rate of coal dust injected at BF-2 in these months happened to be lower by 38.4 and 24.2 kg respectively. However, the summary rate of fuel in the same months went down by 5.53 and 3.9% respectively, while carbon rate decreased by 7.9 and 7.12% respectively.
It should be noted that in spite of poorer, to some extent, operating conditions prevailing at BF-2, its indices in terms of fuel and carbon rates remained to be the best as compared with those at BF-G in all trial periods. This testifies to the fact that under comparatively stable production conditions, even in the short trial periods advantages of BRCU over the chute-type apparatus proved to be quite obvious. The reduction of the summary fuel rate in the trial periods was fluctuating within the range of 2,5 and 6%.

The specific productivity of BF-2 in the trial periods was varying from –6.3 to +4.2%, as compared with the specific productivity of BF-G (see Figure 5). It was connected with variations in the ironmaking conditions in different trial periods.On the whole, the direct comparative analysis of production indices of BF-2 and BF-G proves a high efficiency of the rotary method of charging in terms of summary fuel rates, that were reduced substantially. Fuel was used in a more cost-efficient manner, which is also substantiated by the

analysis of top gas. The utilization of CO in the trial periods was increased by 1-1.5% as compared with the similar indices of BF-G. To analyze the efficiency of BRCU with reference to specific productivity one has to juxtapose the respective data as reduced to comparable conditions. For this purpose the performance data of BF-2 were adjusted by coefficients, given in Table 3. It was the basic indices of heats that were adjusted, such as specific productivity, coke rate, summary fuel rate and summary carbon rate. Having been adjusted, these data were inserted into lines 39,41,43 and 45 of table 2.
BF-2 performance analysis are results adjusted to the conditions, comparable with those at BF-G and are given in Figure 6. As can be seen,
It was the specific productivity of blast furnace that had changed the most after adjustment. In comparison with BF-G index, the specific productivity if BF-2 in the trial periods happened to be higher by 5-17%.

It should be noted that the relative increase in the furnace productivity turned out substantially higher than the relative decrease in fuel rate when a chute-type charger was compared with a rotary apparatus. It is accounted for by the fact that owing to a more uniform distribution of stock in the furnace circumference and lower degree of overcompacting of burden under charging, the rotary distribution of burden would ensure a better gas permeability and hence a higher productivity of the furnace. At rotary distribution, coke rate would be reduced thanks òî a reduced amount of heat to be accumulated for ensuring a stable run of the furnace with burden being distributed more evenly. The total heat needed for reduction and smelting of ore-bearing material would not alter at that, it is only the utilization rate of gas reducing ability which would be improved. Simultaneously with the improving of gas permeability, gas filtration velocity through layers will be also increased, which is likely to reduce somehow the utilization of CO. Probably, due to this fact the positive influence of rotary distribution method upon the summary fuel rate turned out to be less then upon the specific productivity. These obvious facts are given only with one purpose, to prove that the adjustments of ironmaking indices were made in a correct way. Indeed, only after adjusting the parameters of heats and then comparison of BF-2 and BF-G indices under comparable conditions it was possible to register a respectively big changes in the specific productivity vis-a-vis changes in the summary fuel rates. In our investigations carried out earlier at the Bhilai Steel Plant after the implementation of the burden rotary distribution there on BF-3 and at the West Siberia Steel Works, similar regularities in varying specific productivity and relative drop in coke rate were found out [1,2]. Therefore, there is every reason to deduct that as burden is being charged through a rotor, it is gas permeability in the

column that would be improved first of all, thus increasing the utilization of gas reducing ability. The salient features of the burden rotary charging method, referred to in the beginning of this paper, resulted in upgrading the performance indices of BF-2. In the analyzed periods of BF-2 performance Si content in hot metal was by 20 relative percent lower then in BF-G. Therefore, while adjusting the furnaces performance indices to equal conditions, the specific productivity of BF-2 in the trial periods was adjusted to a lower figure, but coke rate, on the contrary, to a higher side. However, as it has been mentioned above, owing to a better quality of stock distribution , the rotary charging method enhances the stability of the furnace run, with a smaller reserve of heat and as a result, with a lower content of Si in hot metal. Therefore, there was no need to adjust Si content in hot metal while determining indices of BF-2. Here it may be noted that slag compositions at BF-2 and BF-G is approximately the same. Obviously, in this case the performance indices of BF-2 under comparable conditions would have been even more attractive.
The results of comparative analysis of BF-2 indices for February and April 2008 prove that the implementation of BRCU made it possible to decrease (as compared with BF-G) the summary rate of fuel per 1 ton of hot metal by 6.22 and 4.21 % respectively and increase the specific productivity of the furnace by 8.86 and 17.3%.
Data for these months are representative and reliable, because they have been taken for a long-lasting periods of operations at BF-2, JSPL.

SUMMARY

The rotary charging unit makes it possible to implement an innovative method of charging stock into the blast furnace By this method material is loaded into the blast furnace in many flows, whereby it is stacked softly in many layers. As it happens, a higher degree of circular uniformity and averaging of each batch of the charged material are achieved A two-year experience of operating the bell-less rotary charging unit at JSPL has proved its benefits as compared with a chute-type chargers. The use of BRCU have testified that the summary rate of fuel was decreased by 4.21 – 6.22% and specific productivity increased by 8.66-17.3% as compared with the furnaces equipped with chute-type chargers. ACS “ROTOR” was developed and implemented to control BRCU. In addition to that a simulation model was developed to control and analyze the process of charging burden material into the blast furnace.